The successful long term performance of metal tubes in structures such as heat exchangers and steam generators depends strongly on operation, maintenance and repair options dictated by performance monitoring. Because of the nature of the application, on-line tube condition monitoring is not practical, and tube assessment is limited to periodic inspection with remote visual and nondestructive examination devices delivered from inside the tubing. In the area of pipeline and storage tank monitoring, a combination pulsing magnetic reluctance coil and ultrasonic transducer, has been used to measure wall thickness and to determine the presence of pits, as taught in U.S. Pat. No. 4,418,574 (Flournoy). Eddy current probe systems generating a plurality of frequencies to detect flaws at different depths in metallic conduits was taught in U.S. Pat. No. 4,855,677 (Clark, Jr. et al.). A variety of ultrasonic probe carriers for nondestructive inspection of long lengths of tubes are also known and taught in U.S. Pat. No. 4,189,944 (Day) and U.S. Pat. No. 4,388,831 (Sherman).
U.S. Pat. Nos. 4,856,337 and 4,955,235 (both Metala et al.) taught a probe carrier system for combined ultrasonic and eddy current inspection of small tubes, primarily metal heat exchanger tubes of steam generators. In these two inventions, the apparatus included a housing which was insertable within the tube to be inspected, and a rotatably mounted probe carrier, where the probes were ultrasonic emitters, and where a pancake eddy current probe was also included for inspection by means of an electromagnetic field. A system for driving such an inspection probe helically within a steam generator tube was taught in U.S. Pat. No. 4,992,735 (Cullen et al.) and U.S. Pat. No. 5,025,215 (Pirl).
The assessment of service induced localized tubing degradation such as intergranular attack and stress corrosion cracking, and the accurate prediction of remaining life requires very sophisticated nondestructive examination, followed eventually by the removal or "pull" of a full tubing cross-section sample which can be examined in the laboratory. This tube pull sample is only available through a sequence of extremely complicated remote cutting and retrieval procedures.
A method of electrical discharge machining for cutting and removal of inside repair liners and tube sections of steam generator tubes, in order to check the stability of the liner under operating conditions, has been taught in U.S. Pat. No. 4,916,282 (Chamming's). The tubing sample is often damaged during the removal procedure making subsequent analyses more complicated. In addition, once a cross-section sample has been removed, the tube involved is removed from service by plugging and no remedial action is possible.
In another area, sled type cutting devices used to re-establish side, branch connections of known location to underground sewer pipe or other buried fluid conduits which have been newly, correctively lined with plastic pipe, by means of pre-programmed pivoting cutting heads, where a video camera permitted observation of the interior of the sewer pipe, are taught in U.S. Pat. No. 4,577,388 (Wood). An improvement of this device is taught in U.S. Pat. No. 5,088,553 (Ralston et al.), where lights are included with the video camera and lateral side branches are precisely located by monitoring return signals from a microwave transmitter/receiver on the cutting device, to precisely operate a rotary cutter head. However, neither ultrasonic nor infrared inspection, to determine where the side connections were located, were considered feasible. Neither of these cutting devices relate to small tubes, and while both cut through plastic pipe, neither can diagnose problems such as pits or cracks in metal tubes nor capture a sample for analysis.
Precision, non-destructive machining of small interior samples from the inside of pipes for retrieval and inspection, has been taught in U.S. Pat. No. 4,845,896 (Mercaldi). There, a section of the tube was sampled by a linearly moveable cutting-sampling apparatus, mounted on wheels and skids, without cutting all the way through the tube wall, utilizing a moveable semi-hemispherical cutter, and leaving a shallow dimple up to about 0.6 mm deep in the interior tube wall. U.S. Pat. No. 4,925,621 (Muth et al.), also taught a linearly moveable cutting-sampling apparatus capable of cutting an interior portion of a tube to a depth of about 0.1 mm and capturing it for analysis. Two curved cutters were used. The tube was not cut all the way through. A first assembly removed an interior surface oxide layer and a second assembly then removed a curved sidewall sample to avoid sharp edges and stress concentration. These inventions solved a number of sampling problems. However, the sample removed in both inventions was very thin, and capable of only limited testing, and neither assembly could selectively locate a tube wall portion of interest, where corrosion, pitting or cracking was diagnosed by the cutting-sampling apparatus itself.
There is a need for a diagnostic sampling apparatus which would be able to diagnose where defects occur in the walls of small metal tubing, and cut away a large interior tube section without also causing retrieval difficulties or requiring removal of the tube from service. It would also be extremely valuable if a pluggable "window" were created through the tube wall after sample capture, allowing various probes to additionally monitor exterior tube conditions near tube sheets or tube support regions, electromagnetically, ultrasonically or visually. It would also be extremely valuable if the sample were large enough to be weldable to a surrogate tube after removal, to permit leak rate and burst testing. It is one of the objects of this invention to provide such an improved, diagnostic/sampling/monitoring combination cutting apparatus, and to provide a method of testing removed samples.